Ion channels are transmembrane proteins which form pores in the membrane which allow ions to pass from one side to the other. Hille, B (ed). Ionic channels of excitable membranes. 1992. They may show ion specificity, allowing specific ions to passively diffuse across a membrane down their electrochemical gradients. Although certain types of channels are on the average open all the time and at all physiological membrane potentials (so-called leak channels), many channels have ‘gates’ which open in response to a specific perturbation of the membrane. Perturbations known to cause opening of ion channels include a change in the electric potential across the membrane (voltage-gated channels), mechanical stimulation (mechanically-gated channels) or the binding of a signalling molecule (ligand-gated channels).
Transporters are proteins in the cell membrane which catalyse the movement of inorganic ions such as Na+ and K+ as well as organic molecules such as neurotransmitters as in the case of so-called re-uptake pumps, e.g. GABA, dopamine and glycine. Two distinguishing features of carriers versus pores are i) their kinetics-movement of ions via transporters is very much slower than the >106 ions per second that is encountered with ion channels and ii) ion channels conduct down electrochemical gradients whereas transporters can ‘pump’ uphill i.e. against concentration gradients (Hille, 1992). The latter process is normally directly dependent upon energy being provided in a stoichiometric fashion.
Ion channel activity has been studied using a technique referred to as “patch clamping.” Hamill, O. P., Marty A., Neher, E., Sakmann, B. & Sigworth, F. J. (1981). Improved patch-clamp techniques for high-resolution current recording from cells and cell-free membrane patches. Pfluger's Archives, 391, 85-100. According to this technique a small patch of cell membrane is generally isolated on the tip of a micropipette by pressing the tip against the membrane. It has been suggested that if a tight seal between the micropipette and the patch of membrane is established electric current may pass through the micropipette only via ion channels in the patch of membrane. If this is achieved the activity of the ion channels and their effect on membrane potential, resistance and current may be monitored. If the electric potential across the membrane remains constant the current supplied to it is equal to the current flowing through ion channels in the membrane. If ion channels in the membrane close, resistance of the membrane increases. If the current applied remains constant the increase of resistance is in direct proportion to an increase of electric potential across the membrane.
Many drugs are known to exert their effect by modulation of ion channels, but the development of novel compounds acting on them is hampered considerably by the difficulty of screening at high-throughput rates for activity. Conventional electrophysio-logical methods such as patch or voltage clamp techniques provide definitive mechanistic information but suffer from the problem that they are unsuited to the rapid screening of test compounds.
WO96/13721 describes apparatus for carrying out a patch clamp technique utilized in studying the effect of certain materials on ion transfer channels in biological tissue. It discloses patch clamp apparatus utilizing an autosampler, such as those utilized with HPLC apparatus, to provide a higher throughput than may be achieved by the conventional patch clamp technique. This apparatus suffers from the problems that it merely semi-automates the drug delivery system, not the patch clamp recording. It therefore suffers from the same limitations as traditional patch-clamping with respect to speed of processing compounds and can in no way be considered a high-throughput system. The system still requires linear processing (i.e. processing of data obtained for one cell after another). In direct contrast the invention described herein provides parallel processing and thus genuine high-throughput of compounds.
The term “biological membrane” used herein is taken to include artificial membranes such as lipid bilayers and other membranes known to a person skilled in the art. Within the context of this specification the word “comprises” is taken to mean “includes” and is not intended to mean “is limited to only”.